The linings of the gastrointestinal tract rapidly regenerate and the balance between growth control, cell migration and differentiation needs to be tightly controlled. Our studies have identified Protein Tyrosine Kinase 6 (PTK6) as a regulator of growth and differentiation in the intestinal epithelium. PTK6 promotes normal intestinal epithelial cell differentiation in animal models. Our preliminary data indicate that knockdown of PTK6 promotes the epithelial mesenchymal transition (EMT) in human colon tumor cell lines, suggesting that PTK6 is important for maintenance of the epithelial phenotype in colon tumor cells. However in some cases, PTK6 appears to have oncogenic functions; we recently demonstrated that disruption of the Ptk6 gene in the mouse conferred resistance to carcinogen induced colon tumorigenesis, and impaired activation of the STAT3 transcription factor. We hypothesize that PTK6 plays an essential role in maintaining the epithelial phenotype, and while loss of PTK6 may impair colon tumor initiation, reduced PTK6 expression may also facilitate the EMT and promote metastases of colon tumors at later stages. To test this, we propose the following four specific aims: Aim 1) To determine the role of PTK6 in regulating the EMT in different human colorectal tumor cell lines; Aim 2) To examine mechanisms by which PTK6 regulates the EMT. A number of identified PTK6 substrates including Sam68, AKT, ?-catenin, and STAT3 have been implicated in the regulation of the EMT; Aim 3) To determine the effect that PTK6 expression, activity, and intracellular localization has on metastasis of colorectal cancer cells in xenograft models in vivo; and Aim 4) To determine how PTK6 expression correlates with the EMT and metastases in human colorectal tumors. Completion of these aims will provide a comprehensive understanding of the contributions of PTK6 to EMT signaling and colon tumor progression. Colon cancer becomes deadly after it metastasizes. Understanding the functions of PTK6 in the normal colon and in colon cancer initiation and progression will be critical for evaluating its potential as a therapeuic target.